The present invention generally relates to a surround device which realizes a three-dimensional sound field by connecting a plurality of speakers, and more particularly, to a surround device provided with boost means for boosting particular frequency components of particular channel signals.
As is conventional, the Dolby Stereo method has been adopted as a two-track sound format such as movies. The Dolby Stereo method uses audio signals put together in two input channels to separate and reproduce signals into four output channel audio signals (left L, center C, right R, and surround S).
For example, an encoder adopting the Dolby Pro Logic method which is known as the two-track sound format described above will be explained with reference to FIG. 1. In the figure, four separate signals (left L, center C, right R, and surround S) are input into the encoder to create two final output signals of an L total (Lt) and R total (Rt). The outline of this signal processing will be explained as follows. After an audio input signal C as a center channel is level-attenuated by an attenuator by 3 dB, it is added to an audio input signal L for a left channel and an audio input signal R for a right channel, respectively. Further, an audio input signal S for a surround channel is added to these added signals after subjected to following four processes to create two final output signals of an L total (Lt) and R total (Rt):
a. the input signal S is attenuated by an attenuator by 3 dB;
b. further, frequency components from 100 Hz to 7 kHz are extracted with a BPF (band-pass filter);
c. in addition, elimination of noises is performed by a noise reduction circuit (Dolby NR encoder); and
d. additionally, signals with phase differences of plus and minus 90 degrees are created.
Next, when the signals encoded by the signal processing with the Dolby Pro Logic method described above are reproduced as a three-dimensional sound field, the signal processing which is a contrary flow to that of the above mentioned encoder is performed. With this processing, the two input signals of Lt and Rt are separated into four channel signals of a front left channel (Lch), center channel (Cch), front right channel (Rch), and rear surround channel (Sch).
Incidentally, the reproduction for the rear surround channel is usually performed by monophonic using two speakers. In addition, the Dolby AC3 method or the like has been proposed as one of multi-channel surround systems other than the method described above.
Further, in these surround devices, a speech portion of a person such as an actor (especially in movies) is generally designed to be fixed in the center channel (Cch).
In the multi-channel surround devices adopting the Dolby Pro Logic and Dolby AC3 methods described above, the total sum level of all channel outputs other than the center channel becomes low rapidly when speech becomes principal. In such a case, it is consequential that the total sum level of all the sound volume of the channels including the center channel becomes also low abruptly. On the other hand, people's ears have been accustomed to the level before it was lowered, so that consequently it is hard to catch the speech from the center channel until people get used to a sound volume produced only by the center channel, and accordingly this point has been a problem.
In addition, particularly in the Dolby Pro Logic method, the frequency components from 100 Hz to 7 kHz of the audio input signal S for the surround channel, when encoded, has been bandlimited by a band-pass filter (BPF), and in particular, the signal level of the frequency components higher than 7 kHz is attenuated as the frequency thereof goes up.
Therefore, there was a problem that the presence as the three-dimensional sound field of the Dolby Pro Logic method is poorer in comparison to other surround systems, for example, the Dolby AC3 method.